As to finding the leak, I use the flourescent dye method eventually. I say eventually because even if we detect with an electronic detector we want the dye stain to show us how it leaked.

As to whether you can improve the A/C. Is the A/C a factory unit or was it afterinstalled? The 107 bodies worse problem was volume of air delivered. Most hang-on units were added externally to the case, which caused them to be small and not situated to take advantage in the poor airflow condition. If this is the case I would consider a DIY project installing a used factory evap case (w/new evaporator) of whichever system the car has. This would cost a fortune if done professionally, mainly because it is one of the worse evap jobs going. Lots of pitfalls.

I would redo it in R12 no matter what system it was. After doing hundreds of conversions, I find them often less efficient. if its a marginal system in R12 then it is likely to be poor in 134.